1. Field of the Invention
This invention relates generally to powder coating of articles by an electrostatic process, and more particularly to a continuous, automated method and apparatus for massproducing electrostatically coated fasteners, such as screws or the like, where substantially the entire exterior surface of the fastener is provided with uniform coverage of the coating.
2. Description of the Related Art
Coating of articles or components frequently is desirable to provide a protective layer of material on the exterior surfaces of the components for a variety of reasons, including protecting against corrosion. In order to provide such a coating, however, a portion of the component typically remains uncoated.
For example, coating of components can be provided in a batch process by what is known as a "dip and spin" method. With this method a plurality of components are dipped or mixed within a vessel containing the desired coating. Upon removal, the components tend to stick or adhere together, and, when spun and dried, must be separated leaving uncoated portions on the components where they previously were adhered.
Alternatively, a component can be held by a mechanism while the coating is applied. Upon release, however, the portion of the component held by the mechanism remains uncoated.
Uncoated portions on components particularly are a problem with coating fasteners such as screws, nails, rivets or the like. Such fasteners are difficult to hold due to their shapes and typically require coating on their entire exterior surfaces.
Fasteners are either batch processed or held as described above and usually have undesirable uncoated portions thereon. If such fasteners are to be held while the coating is applied, they frequently are held by their heads which do not receive a layer of coating. Uncoated fastener heads are not desirable since in use the heads frequently are exposed to the elements.
An example of a coating apparatus which holds or contacts the heads of fasteners is illustrated in U.S. Pat. No. 5,025,750. That patent discloses a method and apparatus for coating a fastener having a head portion and a shank portion where the fasteners are supported on their head portions, are heated to a desired temperature and the protective coating is applied only to the shank portion of the fastener.
Fasteners or other components also can be coated using an electrostatic process where powder particles are electrostatically charged as they exit a spray gun to form a particle cloud or ion field through which an electrically grounded fastener is passed. The charged powder particles stick to the exposed surfaces of the grounded fastener which then is removed from the cloud for curing and/or further processing.
To electrostatically coat fasteners or other components, at least a portion of the fastener must be held as it is conveyed through the cloud of charged particles. Consequently, the portion of the fastener held, which typically is the head of the fastener, does not get covered with the protective coating.
It therefore is desirable to provide a continuous, automated method and apparatus for coating a fastener or other article with a protective coating, such as by an electrostatic process or other method, where the fastener is held by a conveying mechanism which merely is in contact with a minute portion of the exterior surface of the fastener, such as a tip or point thereof, to enable coating of substantially the entire exterior periphery of the fastener and which can be utilized in mass production.